Trusses are pre-manufactured structural roof components which support the roofing and carry the top floor ceilings. Wood trusses are widely used in single and multi-family residential, institutional, agricultural and commercial construction. Their high strength-to-weight ratios permit long spans, offering greater flexibility in floor plan layouts. They can be designed in almost any shape or size, restricted only by manufacturing capabilities, shipping limitations and handling considerations.
FIG. 1 shows an example of a standard truss known to those of ordinary skill in the art as a Howe truss. The truss 100 is made from three primary components: upper chord elements 102, web elements 104, and a lower chord element 106. The lower chord element 106 is arranged horizontally so that it is adjacent to a ceiling structure, which is represented by a reference line 108. The upper chord elements 102 and the lower chord element 106 are connected end-on-end to form an enclosed structure and the web elements 104 are arranged inside the enclosed structure. The upper chord elements 102, the web elements 104, and the lower chord element 106 are typically made from dimensional lumber and may be connected with interlocking structures, nails, metal plates, or other types of connections.
A common problem associated with trusses made from wood is a phenomenon known as trust uplift. Truss uplift is a deformation of the truss structure, normally caused by a moisture differential between the lower chord element 106 and the upper chord elements 102. Because wood is a hygroscopic material, it can expand and contract due to changes in temperature and/or humidity, resulting in a change in length of the truss components. FIG. 2 shows an example of the truss 100 from FIG. 1 after the components have expanded as a result of exposure to a moisture and/or temperature change. In most roof construction applications, the lower chord element 106 is buried in heavy insulation, which keeps it relatively dry and warm in the winter when the rest of the truss 100 is exposed to a higher relative humidity environment of cold air and moisture. As a result, the upper chord elements 102 and the web elements 104 expand, but the length of the lower chord element 106 remains relatively stable. This results in a change in distance between the lower chord element 106 and the upper chord elements 102. In FIG. 2, the degree of uplift is schematically shown by reference character 110. The greatest amount of uplift is often seen at the center of the truss.
Conditions contributing to uplift may include rain, wind, seasonal changes, or other factors influencing the moisture content of each truss component. In addition, components of truss structures may also be exposed to moisture changes during transportation or construction. Over time, the resulting dimensional changes of the truss components can lead to significant deformation of the overall truss structure. In a residential construction application, the lower chord element 106 often lifts in the winter and lowers again in the spring. When trusses arch up, they take the dry wall and ceiling with them, which can cause visible cracking. As the trusses dry out with the warm summer air, they can drop back down closing most of the cracks. This cracking is upsetting to a homeowner, as most homeowners might assume that there are structural problems with the house.
Both the wood products industry and the construction industry have taken steps to minimize truss uplift and other problems associated with the expansion or shrinkage of truss components. One solution is to avoid connecting the truss directly to the wall partitions. Instead the builder can connect the truss with an ‘L’ bracket (known as a “truss clip”) or strap, which allows vertical movement of the truss. An example of this solution is described in U.S. Pat. No. 5,560,156 and U.S. Pat. No. D318,359, which are hereby incorporated by reference. Another technique commonly practiced by wood products manufacturers is to grade lumber according to its strength and select only the strongest lumber to be sold as the upper chord elements 102 and the lower chord elements 106. Although the purpose of this lumber selection technique relates primarily to strength, selecting stronger chord elements may also reduce uplift.
One drawback of known solutions for truss uplift is that many of them involve alterations to the construction of the house. Additionally many are remedial in nature as opposed to preventative. Therefore, the wood product manufacturer has very little control over the perceived performance of the products sold for truss construction. Thus, there is a need to develop a new method that enables wood product manufacturers to select components for constructing trusses with minimized susceptibility to uplift.